The present invention relates to a stimulator head according to the preamble of claim 1 and a method according to the preamble of claim 5.
Stimulator heads are used in magnetic stimulators as transducers of electric energy which is fed to the stimulator head from the stimulator power supply into magnetic field energy. The magnetic field is generated by means of a coil placed into the body of the stimulator head and, in some constructions, the stimulator head may form an integral part of the magnetic stimulator.
As known from the prior art, biological tissue and other conductive media can be excited by applying thereon an electromagnetic field composed of an electric field E and a magnetic field B. Thus, a plurality of different tissues such as the brain, the peripheral nervous system and the heart may be stimulated by means of an electric field. A suitable electromagnetic field can be induced using a coil placed, e.g., on the object to be stimulated.
The cerebral cortex can be stimulated without health risks and pain by applying with the help of one or more coils placed on the head a strong magnetic field that induces an electric current at a desired point. Activation of nerve cells by means of electric current applied thereon can be utilized in a plurality of different ways. For instance, the stimulation of certain cortical areas triggers the contraction of the muscles controlling the functions of the hand thus permitting the velocities of nerve conduction from the brain to the muscles to be measured. The stimulation of certain other areas can be employed to interfere with the normal function of the brain, e.g., during the execution of a given task thus allowing the cortical areas related to the execution of different tasks to be localized. Furthermore, the stimulation of certain areas of the brain may also have therapeutic effects; for instance, patients suffering from depression have been reported to gain relief from the stimulation of cortical areas of the frontal lobe.
Conventionally, the stimulating magnetic field is generated by means of a coil made from coiled loops of a large-diameter, cooled conductor. In order to generate the desired magnetic field, the magnetic stimulator coil is fed with a current pulse typically having a magnitude of 1-20 kA and a duration of 50-500 xcexcs. The repetition rate of the current pulses may be, e.g, 0.1-50 Hz, and they are capable of inducing very strong forces of a rapid rate of change on the conductors. Due to such forces, the coil will be subjected to a massive mechanical shock emitting an acoustic wave with a spectrum typically including frequencies in the range 1-10 kHz and a sound level reaching up to 120 dB and above. The emitted acoustic wave can cause a hearing damage in the ear at the level of the hair cells of the inner ear. Conventionally, such countermeasures as ear plugs and other kind of hearing-protection devices have been used to guard against the acoustic wave. Despite the use of protective devices, the sound often is experienced annoying to both the patient or test person and the examination session operator. Moreover, the sound emitted in conjunction with the stimulation can activate brain areas associated with hearing, whereby the interpretation of brain activation tests will become more complicated. Particularly in research into the hearing system, the noise emitted by the stimulator coil is most disturbing.
It is an object of the invention to overcome the above-described drawbacks of the conventional techniques and to provide an entirely novel type of stimulator head for a magnetic stimulator and a method of attenuating the acoustic wave emitted by said stimulator coil.
The goal of the invention is attained by essentially housing the stimulator coil elements inside a structure which allows at least the stimulator coil proper to be enclosed by a shell taken to a vacuum. With the provision that the interior of the stimulator coil housing can be taken to an essentially complete vacuum, it is at least in principle possible to make a perfect acoustic shield. However, even such an arrangement in practice allows sound to some extend to be conducted to the surroundings via the suspension structures and current supply conductors of the coil. In addition to air-tightness, the stimulator coil housing must be transparent to magnetic field at least for those parts separating the coil from the object being stimulated.
More specifically, the stimulator head according to the invention to be used in a magnetic stimulator is characterized by what is stated in the characterizing part of claim 1. Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 5.
The invention offers significant benefits. By virtue of the invention, it will be possible to make an essential improvement in the attenuation of the noise emitted by the stimulator toward the ears of the person being examined or the test session operator and in the overall disposition of the test environment. The sound attenuation technique according to the invention is capable of damping noise emission from the stimulator equipment to such a low level as to make it blend with the 50-60 dB background noise level which in most auditory tests is considered acceptably low. Then, the noise emitted by the stimulator coil does not any more cause a disturbance nor risk to the hearing of the test persons or the examination session operating personnel. The accuracy of stimulation tests performed by virtue of the invention on the brain and particularly on the hearing system will also be enhanced, because the invention makes it possible to avoid the undesirable activation of auditory cortical areas in conjunction with the application of the stimulation pulse.